The density of air under standard conditions
is only 1.239 milligrams per cubic centimeter under standard conditions.
Measuring the density is done by finding the mass of an evacuated glass
sphere, letting the air back into the sphere, and finding the new mass.
This is done with the sphere on one pan of an equal-arm balance. The outside
diameter of the sphere is then measured, the thickness of the wall estimated,
as well as the volume of the neck, and the volume of the air computed.

Sometimes smaller, metallic spheres were used in this experiment.
An example can be seen mounted on the vacuum
pump in the Kenyon College collection.

This apparatus at the right is at St. Mary's College in Notre Dame,
Indiana. There is no maker's mark.